A basic principle of orthogonal frequency division multiplexing (OFDM) is dividing a data stream having a high rate into multiple data streams having a slow rate and simultaneously transmitting the multiple data streams using a plurality of carriers. Each of the plurality of carriers is called a sub-carrier. Even though frequency components of the carriers overlap each other, a receiving stage can detect the carriers due to orthogonality between the plurality of OFDM carriers.
In a communication system, devices, which communicate with each other, confirm each other's channel status and set a communication method, thereby raising efficiency of the communication system. OFDM widely uses a channel-priority scheme in which channel statuses of users are exchanged to provide priority of communication between devices having a good channel status. To obtain multi-user diversity when multiple users are present, a channel status of each user should be discerned as accurately as possible. Especially, since multiple users transmit signals using sub-carries in OFDM, even a variation in frequency of a channel as well as a variation in time of a channel should be definitely transmitted to maximize the use efficiency of a frequency channel.
As a method for generating channel information of a mobile station, the average quality of received signals over all channel bands may be generated and encoded for transmission. Alternatively, differential encoding may be applied using correlation with a previous transmission value. Thus the generated digital channel information is transmitted to a transmitting stage and the transmitting stage chooses a user having a good channel status according to the channel information and generates traffic at a next transmit time interval (TTI). The traffic is modified to transmit values for all frequency bands in OFDM. All the frequency bands are not indicated by one value but the frequency bands are divided into given units. The quality of a received signal of the divided unit is individually calculated and is encoded. Alternatively, differential encoding may be implemented using a correlation relationship between the quality of received signals or using correlation with a previous transmission value. The generated digital channel information, i.e., channel quality information (CQI) is transmitted to the transmitting stage and is used to choose users having a good channel status in a frequency band when selecting users at a next TTI.
A receiving stage measures downlink quality and reports a CQI value selected based on the downlink quality to a base station through an uplink control channel. The transmitting stage performs downlink scheduling such as the selection of a mobile station and the allocation of resources according to the reported CQI.
FIG. 1 illustrates an example of a wireless communication system.
Referring to FIG. 1, a wireless communication system 100 may be comprised of a base station (BS) 110, and a plurality of mobile stations (MSs) 120, 130, and 140. The base station transmits and receives data to and from the plurality of mobile stations through a downlink channel 115 communicating commonly with the mobile stations and through uplink channels 125, 135, and 145 communicating individually with the mobile stations.
In a mobile communication system, link adaptation is used to maximize utilization of given channel capacity. The link adaptation provides a method for controlling a modulation and coding scheme (MCS) and transmission power according to a given channel. For a transmitting stage to perform the link adaptation, a receiving stage feeds back channel information.
The channel information, e.g., channel quality information (CQI) or channel status information (CSI) is transmitted to the transmitting stage from the receiving stage. A transmission scheme differs depending on which side starts transmission. Hereinafter, CQI will be described as an example of the channel information.
A channel information transmission scheme may include an event-triggered CQI reporting, periodic CQI reporting, and on-demand CQI reporting. In the event-triggered CQI reporting, if the receiving stage determines a difference between a packet transmitting/receiving scheme and a channel or a channel status becomes different from a previous channel state, the transmitting stage transmits the channel information when abnormality occurs in statistical features of a packet (average packet error rate/bit error rate, response delay, etc.). The periodic CQI reporting serves to transmit the channel information of a user through predetermined resources at regular intervals. The on-demand CQI reporting serves to command the receiving stage to transmit the channel information at a time point demanded by the transmitting stage.